This invention relates to a method of enhancing the tensile strength of carbon fibers and more particularly to a method of enhancing the tensile strength of carbon fibers by the use of bromine.
Early in aerospace applications a surface oxidation of carbon fiber was found necessary in order to realize acceptable interlaminar shear strength, and abrasion resistance in epoxy composites. Typical processes recently reported are: (1) I. L. Kalnin, U.S. Pat. No. 3,723,607 using ozone; (2) D. A. Scola and H. A. Hilton, U.S. Pat. No. 3,660,140 using 70% HNO.sub.3 ; (3) R. A. Cass and S. Steingiser, U.S. Pat. No. 3,627,570 using chemical oxidation. While oxidation processes may facilitate enhanced adhesion, the tensile strength is normally diminished. In addition, it has been determined that prior art carbon fiber composites have relatively low impact strengths which limit their potential uses. Further, it has been determined that carbon fibers of different types have different strength and the need has arisen which requires modification of carbon fibers by surface treatments in order to yield the desired shear behavior of carbon fibers in composites.
Commercial carbon fibers are produced from cellulose and polyacrylo nitrile fibers by a sequence of complex chemical processes. Problems reside in the initial heating in which the nucleation of the carbon networks is controlled by a highly-sensitive oxidationpyrolysis step at about 200.degree.-250.degree.C; this involves complex cyclization processes combined with a chemical conversion to aromatic networks. Further problems reside in the subsequent heating process to higher temperatures during which time-temperature dependent reactions similar to many coking processes take place; which is followed by special heat treatments which may attain elevated temperatures of about 2000.degree.C. Even with today's technology the fiber structure and surface behavior of carbon fibers are not understood.